As a method of generating a stable voltage lower than an input voltage, a non-insulated step-down chopper circuit is widely used. In the step-down chopper circuit, since a switching operation is continuously performed even in a light-load state such as in a waiting time, power conversion efficiency is lowered as the load becomes lighter.
U.S. Pat. No. 5,481,178 discloses a switching power-supply device which detects a light-load state from an error signal generated by comparing an output voltage and a reference voltage and transitions to intermittent oscillation control, in which a period where driving of a switching element is permitted and a period where driving of the switching element is prohibited are alternately repeated in the light-load state.
In this switching power-supply device, by lowering a switching frequency of the switching element as an output current decreases in the intermittent oscillation control, a switching loss and a drive current of the switching element are decreased and thus the efficiency in the light-load state is improved.